Process for sorting motile particles from lesser-motile particles and apparatus suitable therefor

ABSTRACT

Motile particles are sorted from non-motile particles in a microfluidic sorting device wherein a stream of sort fluid containing motile and non-motile particles is caused to flow adjacent a media stream in non-turbulent fashion through a sort channel, during which flow motile particles cross the interface between the adjacent flow streams, entering the media stream, and forming a motile particle-depleted sort stream. The sorting devices are easily and inexpensively fabricated and have numerous uses, in particular sorting of motile from non-motile sperm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/061,482, filed Apr. 2, 2008, which is a divisional of U.S. patentapplication Ser. No. 10/375,373, filed Feb. 27, 2003, which claims thebenefit of U.S. provisional application Ser. No. 60/359,844, filed Feb.27, 2002 each of which is herein incorporated by reference in theirentireties.

This invention was made with government support under HD035125 awardedby the National Institutes of Health. The government has certain rightsin the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a process for sorting motile fromlesser-motile particles, particularly motile and non-motile particles ofbiological origin, and to apparatus suitable therefor.

2. Background Art

The separation, or “sorting” of motile from lesser-motile and/ornon-motile particles has numerous applications, but heretofore has beentechnologically difficult. For example, in analysis of water supplies,it may be desirable to separate motile bacteria and othermicroorganisms, including fungi, algae, etc., from those which arenon-motile. Identification and enumeration of the various species maythus be facilitated. Examples of motile organisms include flagellatedand ciliated bacteria such as C. elegans, and other microorganisms, suchas paramecia and motile plankton. Either the motile species enriched ormotile species-depleted samples, or both, may be independently analyzed,cultured, etc.

An especially significant application is the sorting of sperm cells. Forexample, in the case of in vitro fertilization, if the donor's spermcount is low, and especially if contaminated with non-motile sperm,deformed sperm of lesser motility than the desired viable sperm, andother cells and seminal debris, the success rate is raised considerablywhen the motile sperm are used substantially for fertilization attempts.For example, avoidance of anueploid sperm or DNA fragmented sperm isparticularly desirable. In many endeavors, it is desirable to be able todirect the gender of the offspring, for example when the birthing ofmilk cows is desired. In such cases, it would be advantageous to be ableto sort the X- and Y-chromosome containing sperm based on their knownmotility differences.

Sperm cells from donors with oligozoospermia (low sperm count) havepreviously been concentrated and to some degree separated from cells anddebris having different sizes and/or densities by centrifugation.However, this technique allows incorporation of non-gametes into theenriched sperm sample. These non-gametes, however few there are, releaseoxygen radicals which are detrimental to continued sperm viability.Moreover, centrifugation is a brute force technique which damagessignificant numbers of sperm, particularly at the mid-piece and tailregions.

So-called “swim up” techniques are also known, but isolation of the mostviable sperm is challenging. S. Smith et al., FERTIL. STERIL. 1995, 63,591-97. Thus, doctors frequently resort to hand sorting through deadsperm and debris to find sperm which are motile and of distinctmorphology, a very time-consuming process.

Applications in biogenetics (biotechnology) also frequently requireseparation of particles based on their motility. In non-biologicalapplication, separation of working microrobots (which are motile) fromnon-working microrobots is a possible application.

SUMMARY OF THE INVENTION

Sorting of motile and non-motile or lesser-motile particles isaccomplished by establishing a non-turbulent and preferably laminar flowstream (“sort stream”) containing motile and non-motile or lesser-motileparticles to be sorted, and contacting this sort stream with a secondnon-turbulent and preferably co-laminar media flow stream (“mediastream”), providing an exit stream for at least a portion of a motileparticle-enriched media flow stream, and an exit stream for a motileparticle-depleted sort stream. The mobility of the motile particlesallow them to enter the media stream along the interface between themedia and sort streams, while non-motile or lesser-motile particlesremain substantially within the sort stream. Apparatus suitable for usein the process provide for at least one sort stream inlet, at least onemedia stream inlet, at least one sort channel, at least one motileparticle-depleted sort stream outlet, and at least one motileparticle-enriched media stream outlet. The apparatus are preferablyrelatively small devices prepared by micromachining or polymer castingtechniques, and preferably contain all necessary functionalityintegrated onto a single “chip.”

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simple embodiment of a motile particle sortingdevice of the present invention;

FIG. 2 depicts sorting of motile from non-motile particles in a sortchannel of a device of FIG. 1;

FIG. 3 illustrates one embodiment of series connected sorting devices;

FIG. 4 illustrates a further embodiment of a sorting device withmultiple media inlets;

FIG. 5 illustrates in schematic form a further embodiment of a sortingdevice in accordance with the present invention;

FIG. 6 illustrates in schematic form a further embodiment of a sortingdevice in accordance with the present invention;

FIG. 7 illustrates a device similar to that of FIG. 1, in perspective;and

FIG. 8 illustrates sorting efficiency of a device similar to that ofFIG. 1 in sorting sperm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description of the invention may be facilitated by reference to FIG.1 which represents one relatively simple embodiment of an apparatuswhich may be used for practicing the subject invention, and by referenceto FIG. 2 which illustrates sorting of motile from non-motile particles.Following these descriptions, additional details of the functioning ofthe apparatus, its geometry, the nature of fluids and fluid flow, etc.,will be explained in greater specificity.

The device of FIG. 1 is shown in schematic form in plan, i.e. as viewedfrom above. The device 1 embodiment consists of a motile particle sortstream inlet 2 (or motile particle supply reservoir serving as aninlet), a media stream inlet 3 (or media reservoir), a motileparticle-depleted sort stream outlet (or reservoir) 4, and a motileparticle-enriched stream outlet (or reservoir) 5. Between the inlets 2and 3 and the outlets 4 and 5 is located a sort channel 6. Connectingthe sort channel 6 to the respective inlets and outlets are sort streaminlet channel 7, media stream inlet channel 8, motile particle-depletedsort stream outlet channel 9, and motile particle-enriched media streamoutlet channel 10. The width of the sort stream channel must be largeenough to allow the particles of interest to pass through effectivelywithout blockage, as is also true of both outlet streams. In general,the inlet streams and outlet streams will have a cross-sectional arealess than the sort channel. The relative cross-sections will bedependent on the flow volume and flow rates of the various streams. Thelinear flow rates are preferably similar, although the relative flowrates are only limited by the occurrence of mixing between the sortstream and the media stream. Depending upon numerous factors such as theviscosities of the media and sort streams, the motility of theparticles, and the presence or absence of particles or debris ofdifferent size than the particles desired to be sorted, the volume ofthe media stream may be less than, substantially the same as, or greaterthan the volume of the sort stream over any section of the sort channel.

The bulk of the description which follows is described in relation tosorting of sperm, although the same principles apply to other motile andnon-motile particle sources.

In operation, a supply of sperm is introduced into sort stream inlet 2and caused to flow toward motile particle-depleted sort stream outlet 9,initially through channel 7, then through sort channel 6, and next tooutlet channel 9. A media supply stream compatible (i.e. notdestructive) of the sperm to be sorted is introduced into media streaminlet 3 and caused to flow through channel 8 into sort channel 6,through channel 10, and into motile particle-enriched media outlet 5. Atthe confluence of channels 7 and 8, a non-mixing, and preferably laminarflow is created, such that the sort stream and media stream flow inparallel through the sort channel. Non-motile (or lesser-motile)particles tend to remain in the sort stream, while motile particles moveabout randomly and enter the media stream. As a result of this randommovement, the sort stream becomes depleted of motile sperm, while themedia stream becomes increasingly enriched.

The invention may further be described broadly with reference to FIGS.2a, 2b, and 2c , which illustrate pictorially the separation of motilefrom non-motile sperm and other non-motile particles in the sort channelof the device of FIG. 1. In FIG. 2a , the sort channel 6 is shown,beginning at the point of confluence of the sort stream 11 and the mediastream 12. The sort stream 11 contains motile 13 and non-motile sperm 14as well as other non-motile particles, here designated as “round cells”15. Note that the size of the media stream in plan, and hence itsvolume, is considerably greater than the sort stream. Since sperm (andsimilarly, other motile particles) assume an essentially randomdistribution in the total liquid within a short period, a larger mediastream volume will necessarily contain a larger fraction of total motilesperm 13.

In FIG. 2b , the randomization of motile sperm between the two streamshas begun, and continues until the desired degree of randomization hasbeen achieved. This degree of randomization is preferably such that theconcentration of motile sperm in the media phase per unit of volume isthe same or greater than the concentration per unit volume in the sortstream. Note that the sort stream and media stream are maintained asseparate streams, each preferably exhibiting laminar flow, and having acommon boundary, or interface, 16. Greater concentration of motileparticles in the media stream over the amount dictated by purerandomization may be achievable by employing a media stream in which themotile particles have increased mobility, i.e. by selecting a mediastream less viscous than the sort stream, or by including additiveswhich increase mobility of motile sperm relative to non-motile or poorlymotile sperm.

In FIG. 2c , the motile sperm-enriched media stream is harvested bydiverting it to flow into the motile particle-enriched channel 10, whilethe now motile particle-depleted sort stream continues through channel 9into outlet 4.

The diverting juncture 17 which separates the motile particle-depletedsort stream from the motile particle-enriched stream may be of anygeometry which avoids substantial mixing of the streams at this point.The juncture 17 may be positioned, for example, to provide forsubstantially the same outlet channel configuration (i.e. height, width)of the sort stream inlet, at this point. To minimize contamination ofthe media stream by non-motile sperm, the juncture 17 may also beconfigured such that a small portion of the media stream is alsodirected to the motile particle-depleted sort stream outlet 9. In thiscase, a modest loss of motile sperm will occur, however, the probabilitythat non-motile sperm may enter the motile sperm-enriched media streamwill be lessened as a result.

The nature of the sort stream is not critical. The sort stream may be abiologically derived stream such as semen, or may be washed, diluted,may be treated with additives, stains or fluorescing dyes, viscositymodifiers, may be buffered, etc., so long as the treatment does notimpair the viability of the desired exit stream (motileparticle-depleted or motile particle-enriched). If separation but notviability is the aim, for example with bacterial samples, the number ofpotential modifications of the sort stream are enlarged. The sort streammay also be a previously motile particle-depleted sort stream or motileparticle-enriched media stream. For microrobotic devices, the sortstream liquid may be any which does not impair the functioning of thedevice, for example water, alcohols, ketones, glycols, esters,hydrocarbons, etc. With biological samples, water-based sort streams areordinarily used.

The media stream may be selected with the same considerations in mindwhich are applied to selection or modification of the sort stream. Insome cases, the sort stream may be water, but for biological systems, itis typical to employ streams which maintain or enhance biologicalactivity, such as physiological saline, buffered saline, nutrientbroths, and the like. In the case of human sperm, the preferred media isHEPES buffered human tubal fluid.

The nature of the media fluid and the sort fluid may be selected, ifpossible, to avoid interfacial mixing due to osmotic effects. This isthe case, for example, when the base fluid (e.g. water) of both the sortand media streams have substantially the same amounts of solubleingredients such as salts, acids, bases, buffers, dissolved organicmaterial, and the like. The fluids may also be selected, when possible,to avoid interfacial mixing by diffusion. However, complete absence ofany diffusion is an unlikely goal in this respect.

The relative fluid volumes may be selected with respect to the desireddegree of incorporation of the motile particles within the media phase.For the highest degree of incorporation, the media volume should belarge with respect to the sort fluid volume. However, proportionatelysmaller media volumes may also be used, particularly when sequential(serial) sorting is performed. Ratios of media fluid to sort fluid offrom 1:1000 to 1000:1 are preferably used, more preferably 1:100 to100:1, and most preferably within the range of 1:10 to 10:1. For typicalapplications, the ratio of media volume to sort fluid volume is withinthe range of 1:1 to 3:1. The media fluid volume is most preferablyhigher than the sort fluid volume.

The volumes referred to here are the volumes at a given cross-section ofthe sort channel. For example, a sort channel which is rectangular inshape having dimensions of 100 μm×200 μm will have a “transverse volume”of 2×10⁴ μm². This “transverse volume,” actually a cross-sectional area,can be converted into true volume by multiplying by channel length or anincrement thereof. Thus, the same rectangular channel previouslydescribed and having a transverse volume of 2×10⁴ μm² will have anactual volume over a 100 μm length of 2×10⁶ μm³.

The linear flow rates of the sort fluid and media fluid are preferablysubstantially the same, i.e. within a range of flow rates of 1.5:1 to1:1.5. If the linear flow rate of the media fluid is greater than thatof the sort fluid, correspondingly less transverse volume of media fluidcan be used for the same degree of motile particle incorporation. Flowis preferably concurrent, although counterconcurrent flow is alsopossible provided that interfacial mixing is not exacerbated beyond thatwhich facilitates the desired degree of depletion/enrichment of the sortand media fluids.

The interface between the sort and media fluids is preferably asubstantially non-mixing interface. By “non-mixing” is meant an absenceof mixing which occurs due to excessive turbulence between the twofluids. For example, it is most desired that parallel, concurrent,laminar flow take place such that a substantially “static” appearinginterface is obtained, as opposed to an interface which exhibits waves,currents, eddys, and the like. Turbulent flow generally results inpartial to full mixing of the streams, rendering depletion/enrichment ofmotile particles less efficient or even completely impossible. Thetheoretically best resolution of motile particles occurs when astatic-appearing interface or “streamline” is created where interfacialmixing occurs only due to diffusional and osmotic effects. However, itwould not depart from the spirit of the invention to allow someturbulence along the interface. The turbulence is excessive when thedesired degree of resolution cannot be obtained, even with multiplestages of devices. The turbulence, expressed as a Reynolds number,should in any case be less than 2000, more preferably less than 100, yetmore preferably less than 10, and most preferably 1 or less. Highperformance devices such as those illustrated by example herein, exhibita Reynolds number of approximately 0.1.

The nature of the interface, i.e. its degree of turbulence, may beassessed by the degree of resolution. However, the turbulence may alsobe assessed in numerous additional ways. For example, in PDMS devices asdescribed hereinafter, the optically transparent nature of the deviceallows the interface itself to be observed microscopically, for exampleby coloring one or both of the fluids and observing the interface by thechange of color at the interface. By conventional optical techniques,the interface between media of differing refractive index are alsoeasily observed. The degree of mixing of the sort and media streams mayalso be monitored by introducing a taggant, i.e. a radioactive solublecompound or non-motile particle, a visual or fluorescent dye, etc., intoone stream but not the other. Appearance of the taggant in the outletstream of the stream initially containing no taggant provides evidenceof interfacial mixing, either of a turbulent kind, or by diffusion orosmosis. Some incorporation due to the latter two effects is expected,but is also expected to be quite minimal. An incorporation of 50% of thetaggant into the non-tagged stream essentially constitutes completemixing. Mixing of less than 20% of the taggant into the non-taggedstream, preferably less than 10%, more preferably less than 5%, and mostpreferably less than 1% is desired. So long as the Reynolds number iskept reasonably low, the degree of turbulence will be satisfactory. Aflow which satisfies the above criteria is termed a “substantiallynon-turbulent flow” herein. It should be noted that concurrent flowstreams exhibit much less turbulence, and hence interfacial mixing, thancountercurrent flow streams.

Provided the fluid flow rate meets the non-turbulent requirements justdescribed, the rate itself may vary widely. The walls of the sortingdevice also create the possibility for turbulence, since they are staticwith respect to the fluid flow. The effect of the walls will be mostimportant when narrow channels are employed, and particularly at thewalls which abut the narrower of the sort or media streams. Since thedevices of interest are rather small and have rather small channels,linear flow rates of less than 10 cm/s, preferably less than 10 mm/s arepreferred. Flow rates of between 0.1 mm/s to 10 mm/s are particularlypreferred. The low end of linear flow rate is determined by the mixingof non-motile particles from the sort stream into the media stream byBrownian motion. For example, at a flow rate of zero, with identicalbase fluid compositions (e.g. buffered saline), distribution ofnon-motile particles into the media phase would eventually be completeover time such that their concentrations become identical. The higherthe flow rate, the less Brownian redistribution of non-motile particleswill occur. The upper limit of the flow rate is reached when theinterfacial flow becomes turbulent, as evidenced by a high degree ofmixing.

Determining the relative flow volumes, relative flow rates, and absoluteflow rates of any given stream can be routinely accomplished by oneskilled in the art by simple calculations and/or measurements ofresolution, for example by varying the respective rates and volumes anddetermining the relative enrichment and depletion of particles betweenthe sort and media streams.

The geometry of the devices can vary. Sort channel length, for example,is generally a function of the rapidity at which motile particlesrandomize themselves between the two phases, and the flow rates. Forexample, at a given flow rate, motile particles which have limitedmotility will require a longer sort channel, while at a given sortchannel length, less motile particles will require a slower rate offlow. Interfacial surface area also effects the geometry of the device.For example, flat rectangular sort channels with one fluid locatedparallel to and abutting a channel face of greater dimension, and withthe other fluid adjacent, will exhibit faster randomization and thusrequire less sort channel length than the same channel when the firstfluid is located parallel to and abutting a channel face of lesserdimension. In the latter case, the interfacial area is much reduced ascompared to the former.

While it is theoretically possible to construct devices of macroscopicsize, even of greater than 10 cm in length, for most purposes, the sortchannel will be quite short, in almost all cases less than 2-5 cm, andfor most devices, in the range of 100 μm to 1 cm. For sperm sorting, forexample, a sort channel length of 5000 μm (5 mm) has proven quitesatisfactory. In staged devices, shorter sort channel lengths may bedesirable.

The cross-sectional geometry of the sort channel is not critical.Square, rectangular, ellipsoidal, circular, trapezoidal, triangular orother cross-sections may be used. For ease of manufacturing,non-undercut channels such as square, rectangular, triangular,trapezoidal, and half-round or half-elliptical sections are preferred.These shapes are preferred, for example, when neat casting or solutioncasting methods of construction are employed. In the case ofconstruction by stereolithography techniques (“SLA”), more complexshapes can easily be fabricated. Complex shapes with undercut channelscan also be provided by casting techniques when the device is cast insuccessive layers which are then attached together, for example bybonding. However, the channel width must be such that both the mediastream and sort stream can both incorporate particles. For human spermsorting, for example, a substantially rectangular channel with a heightof 50 μm and a width of 500 μm has proven satisfactory. For a point ofreference, human sperm have a head of about 2.5 μm in diameter and about5 μm long, and are about 60 μm in overall length.

The cross-sectional areas and hence size of the supply channels andoutlet channels are generally smaller than those of the sort channel.The minimum size of the sort stream inlet channel is dictated by thesize of the particles which are present in the sort stream. Preferably,the sort stream channel provides a free channel from 3 to 10 times thesize of the particles expected to be contained therein. The sameconsiderations apply to the size of the media stream outlet channel, butnot necessarily to the media stream inlet channel. Preferably, the sortstream inlet and outlet channels will have comparable sizes, although insome instances, as described earlier, it may be desirable that theoutlet channel is larger than the inlet, thus incorporating a portion ofthe media stream into the sort stream. For sperm sorting, a rectangularsort stream inlet channel of 50 μm height, 100 μm width, and 5000 μmlength has proven satisfactory.

The length of the various inlet and outlet channels is not critical. Itis preferred that at least the inlet channels have some substantiallength, to encourage formation of a laminar flow stream prior to theconfluence of the sort and media stream channels. In general, moreviscous fluids will not require as long a channel length as less viscousfluids. In some cases, the inlet channels may be completely dispensedwith, i.e. the sort stream inlet (or reservoir) and/or media streaminlet (or reservoir) may feed directly into the sort channel. For mostpurposes, however, and to facilitate construction of sorting devices, itis preferable that inlet channels be employed. For the sperm sortingdevice described later, for example, inlet channel lengths of about 3 mmhave proven satisfactory.

The junction 18 (FIG. 1) of confluence of the sort and media streams ispreferably configured to encourage a smooth joining of the fluid streamswithout excessive mixing. In general, therefore, the junction will be arelatively acute angle. The included angles between the sort streaminlet channel and the sort channel and between the media stream inletchannel and sort channel may be the same or different, i.e. the devicesare not necessarily symmetrical. The same considerations apply to thejunction 17 where the sort stream and media stream are separated, or“diverted” from each other. However, it is preferred that the sortstream inlet channel, sort channel, and sort stream outlet channel besubstantially linear to provide as little disturbance of the non-motileparticles in the sort stream as possible.

The material of construction of the sorting devices may be any suitablematerial, and the fabrication of the device may involve any fabricationprocess. For example, devices may be micromachined chemically by etchingof glass, silica, silicon, metals, or by solution etching of polymers,etc. The devices may also be individually fabricated by knownstereolithography techniques. The devices may be injection molded ofmoldable polymers, for example silicone rubber, thermoplasticpolyurethane, polyethylene, polypropylene, polytetrafluoroethylene,polyvinyl chloride, polyvinylidene chloride, polyamide, polyester, andthe like.

It is at present preferable to cast the sorting devices by supplying anegative “master” and casting a castable material over the master.Preferred castable materials are polymers, including epoxy resins,curable polyurethane elastomers, polymer solutions, i.e. solutions ofacrylate polymers in methylene chloride or other solvents, andpreferably, curable polyorganosiloxanes, most preferably for costreasons, polyorganosiloxanes which predominately bear methyl groups,such as polydimethylsiloxanes (“PDMS”). Curable PDMS polymers are wellknown and available from many sources. Both addition curable andcondensation-curable systems are available, as also are peroxide-curedsystems. All these PDMS polymers have a small proportion of reactivegroups which react to form crosslinks and/or cause chain extensionduring cure. Both one part (RTV-1) and two part (RTV-2) systems areavailable. Addition curable systems are preferred when biologicalparticle viability is essential.

In many instances, transparent devices are desirable. Such devices maybe made of glass or transparent polymers. PDMS polymers are well suitedfor transparent devices. A benefit of employing a polymer which isslightly elastomeric is the case of removal from the mold and thepotential for providing undercut channels, which is generally notpossible with hard, rigid materials. Methods of fabrication ofmicrofluidic devices by casting of silicone polymers is well known. See,e.g. D. C. Duffy et al., “Rapid Prototyping of Microfluidic Systems inPoly(dimethylsiloxane),” ANALYTICAL CHEMISTRY 70, 4974-4984 (1998). Seealso, J. R. Anderson et al., ANALYTICAL CHEMISTRY 72, 3158-64 (2000);and M. A. Unger et al., SCIENCE 288, 113-16 (2000).

The nature of the channel and reservoir walls of the devices may beselected in view of the application of the device and the fluidscontemplated for use therein. For example, the walls may be hydrophobicor hydrophilic, or some portions of the device may be hydrophobic whileother portions are hydrophilic. In addition, the walls may be treated orderivatized to modify their surfaces with biologically compatible orbioactive coatings, or to provide chemical functionality. For spermsorting, coating the channels with bovine serum albumin (BSA) has provenuseful in improving liquid flow within the channels and to minimizenon-specific adsorption of cells to channel walls.

Fluids may be supplied to the inlets or inlet channels of the device byany suitable method. Fluids may, for example, be supplied from syringes,from microtubing attached to or bonded to the inlet channels, etc. Inpreferred devices, the sort stream inlet and media stream inlet are inthe form of “on-chip” reservoirs capable of holding and supplying therequisite amounts of liquids. These reservoirs may be filled by syringe,pipet, etc.

Fluid flow may be established by any suitable method. For example,external micropumps suitable for pumping small quantities of liquids areavailable. Micropumps may also be provided in the device itself, drivenby thermal gradients, magnetic and/or electric fields, applied pressure,etc. All these devices are known to the skilled artisan. Integration ofpassively-driven pumping systems and microfluidic channels has beenproposed by B. H. Weigl et al., PROCEEDINGS OF MICROTAS 2000, Enshede,Netherlands, pp. 299-302 (2000).

Preferably, however, fluid flow is established by a gravity flow pump,by capillary action, or by combinations of these methods. A simplegravity flow pump consists of a fluid reservoir either external orinternal to the device, which contains fluid at a higher level (withrespect to gravity) than the respective device outlet. Such gravitypumps have the deficiency that the hydrostatic head, and hence the flowrate, varies as the height of liquid in the reservoir drops. For manydevices, a relatively constant and non-pulsing flow is desired.

To obtain constant flow, a gravity-driven pump as disclosed in publishedPCT application No. WO 03/008102 A1 (Jan. 18, 2002), herein incorporatedby reference, may be used. In such devices, a horizontal reservoir isused in which the fluid moves horizontally, being prevented fromcollapsing vertically in the reservoir by surface tension and capillaryforces between the liquid and reservoir walls. Since the height ofliquid remains constant, there is no variation in the hydrostatic head.

Flow may also be induced by capillary action. In such a case, fluid inthe respective outlet channel or reservoir will exhibit greatercapillary forces with respect to its channel or reservoir walls ascompared to the capillary forces in the associated inlet channel orreservoir. This difference in capillary force may be brought about byseveral methods. For example, the walls of the outlet and inlet channelsor reservoirs may have differing hydrophobicity or hydrophilicity.Alternatively, and preferably, the cross-sectional area of the outletchannel or reservoir is made smaller, thus exhibiting greater capillaryforce.

Most preferably, integrated, on-board reservoirs which serve as constantflow rate gravity-driven pumps and which also exhibit a difference incapillary forces between inlet and outlet are used. Flow in such devicesmay begin as soon as the devices are filled with liquid or when blockingvalves or plugs are opened, or may be initially assisted by a pressuredifferential between the inlet and outlet, for example by applyingsuction briefly to the outlet.

Multiple devices may be connected in many ways to effect complexseparations, to provide enhanced yield, to provide increased resolution(sorting efficiency) or any combination of these. In addition, multiplesort and/or media streams may be employed. When multiple sort or mediastreams are used, the sort streams may be the same or different, as maybe the media streams.

For enhanced efficiency, for example, the motile particle-depleted sortstream outlet of a device such as that depicted in FIG. 1 may beconnected to the sort inlet of a second device, this second device alsohaving a media supply. As a result of this sequential contact with twomedia streams, the sort stream will be further depleted of motileparticles. The motile particle-enriched streams from both devices may becombined. Use of several sequential stages in this manner allows forvirtually 100% recovery of motile particles. Preferably, when multipledevices are employed, they are fabricated on the same structure withintegral connecting channels. One such device is shown schematically inFIG. 3.

In FIG. 3, the series configured two-stage motile particle sorterconsists of a single sort fluid reservoir 20, connected to first sortchannel 22 by sort stream channel 21. The first stage also consists offirst media supply reservoir 23, media stream channel 24, motileparticle-enriched first media channel 25, and motile particle-enrichedfirst media reservoir 26. The motile particle-depleted sort outletstream from the first stage flows through connecting passage 27 to serveas the sort stream inlet to the second stage sort channel 28. Secondmedia reservoir 29 supplies media to the second stage sort channelthrough media inlet channel 30. Sort fluid further depleted of motileparticles exits the device through channel 31 into motileparticle-depleted sort stream reservoir 32, while a second stream ofmotile particle-enriched media fluid exits the sort channel throughmedia outlet channel 33 and into reservoir 34. The two motileparticle-enriched media reservoirs 26, 34 can be connected to a commonexit channel or reservoir, optionally through valved passages, or may beemptied manually, e.g. using a syringe or pipet, and their contentscombined, if desired.

Additional devices are shown in FIGS. 4, 5, and 6. In the device of FIG.4, two media supply reservoirs 41 supply media fluid to the device 40,motile particle-enriched media being collected in the two media outletreservoirs 42. A single sort fluid reservoir supplies fluid containingmotile and non-motile particles from sort fluid reservoir 43, and themotile particle-depleted sort fluid is collected in sort fluid outletreservoir 45 after passing through sort channel 44. In this case, acentral sort stream 46 is flanked on each side by media streams 47.

FIGS. 5 and 6 are schematics of multiple stage devices which rely onalternative connections of various flow paths to improve one or moreaspects of the sorting process. In both Figures, double lines representsort channels. The device of FIG. 5 is capable of not only sortingmotile from non-motile particles, but also into fractions of differentmotilities, and has three sort channels. The device of FIG. 6 splits theoutlet of a single sort channel into fractions, the furthest away fromthe sort stream containing proportionately more of the particles withhighest motility. As can be seen, the present devices can be configuredsimply or with great complexity. Devices may also operate in parallel,series parallel, or other modes. Parallel processing may be desired forsorting larger samples, or to measure sorting efficiency, etc., whilecomparing different media fluids. Such comparisons are morestatistically accurate when measurements are made in a single device.

While much of the description herein refers to separation of motile fromnon-motile particles, the subject invention processes and devices arealso suitable for separating motile particles of differing motility. Themost motile particles will enter the media stream at a higher rate thanparticle's of lesser motility. The residence time in the sort channel ispreferably selected such that the most motile particles will assume arandom or near random distribution in the total fluid. In contrast toseparation of motile from completely non-motile particles, however,where additional sort channel length can be tolerated, and distributionof non-motile particles into the media stream is due substantially onlyto Brownian motion and to turbulence and like effects, when motile andlesser motile particles are separated, the lesser motile particles willalso assume a random distribution given sufficient time. The sortchannel length must be adjusted downward such that this cannot occur.The media stream will be enriched with both motile and lesser motileparticles, but will be correspondingly more greatly enriched by theparticles of greater motility. Multiple sequential processing of a firstmedia stream (serving as the sort stream to a further stage) will causehigher resolution between the differently motile particles. Second andfurther sorting of the sort streams and their subsequent treatment inlike fashion will increase the yield.

Example 1

A microfluidic sperm sorting device was prepared from Dow CorningSYLGARD® 184 curable silicone resin, using the soft lithographytechnique described by D. C. Duffy et al., cited previously. The curablePDMS was cast onto a master having the desired reservoir and channelfeatures as protuberances. The cast PDMS sorting devices were plasmaoxidized to seal the open channel side of the casting to a glass coverslide. Channels and reservoirs were coated with 1% bovine serum albuminfraction V from Sigma, dissolved in phosphate buffered saline (PBS) fromInvitrogen Corporation. The entire device was approximately 6 mm thick,exclusive of the cover slide, and somewhat larger than a U.S. pennycoin. A perspective view of the device is shown in FIG. 7.

In FIG. 7, the PDMS casting is transparent, and only the reservoirs andchannels are depicted. The cover slide would be bonded to the bottomplane of the device. The numerals are the same as those of FIG. 1. Thechannels are rectangular in cross-section, with a channel height of 50μm, while the reservoirs are roughly semi-circular. Both inletreservoirs 2 and 3 are approximately 3 mm in height, while the outletreservoirs are approximately 2 mm in height. The inlet and outletchannels 7, 8, 9, 10 are about 5000 μm long. The sperm inlet channel 7and the motile depleted sperm outlet channel 9 have a width of 100 μm,while the media inlet channel 8 and outlet channel 10 have a width ofabout 300 μm. The sort channel 6 has a width of 500 μm and a length of5000 μm.

Semen samples were obtained with institution Review Board approval frommen undergoing infertility evaluation. Sorting tests were performedusing washed semen samples. In the order listed, 60 μL of processingmedia was added to the media inlet reservoir, 50 μL of a washed semensample to the sample inlet reservoir, and 2 μL of media to each of theoutlet reservoirs. Sperm sorting yields were calculated taking thesedilution factors into account. The numbers of motile sperm weredetermined by a Makler Counting Chamber (Sefi-Medical Instruments,Haifi, Israel). For visualization of membrane-compromised sperm, whichgenerally corresponds to non-motile sperm, 3 μL of propidium iodide(Molecular Probes, www.probes.com, 60 mM dissolved in processing media)was added to sperm samples prior to sorting. A Texas Red filter set (577nm excitation, 620 nm emission) was used to view red fluorescence fromstained cells. An inverted microscope (NIKON TE 300, www.nikon-usa.com)with a CCD camera (Hamamatsu ORCA-100, www.hamamatsu.com) was used tocapture images and record movies.

The sorting device uses a sorting system where non-motile sperm flowalong their initial streamlines and exit one outlet whereas motile spermcan deviate from their initial streamlines and exit through a differentoutlet. This sorting mechanism is related to the “filtering” mechanismused in an “H-filter” where rapidly diffusing small molecules exitthrough a different outlet from larger molecules and particles thatdiffuse more slowly. The difference between the two devices is that thesorting device of the present invention takes advantage of activemovement of cells whereas an H-filter takes advantage of passivediffusion of particles. This type of sorting is possible because insmall channels, multiple laminar streams can flow parallel to each otherwith no turbulent mixing at the interface between the streams. TypicalReynolds Numbers for the flow of sperm sample and media inside thesorting device were on the order of 0.1. Non-motile human sperm,approximately 60 μm in length, and non-motile particles on the sameorder of magnitude in size diffuse slowly (D=1.5×10⁻¹³ m²/sec; 690 secto diffuse 10 μm) and remained within their initial streamlines. Incontrast, motile human sperm swim at velocities greater than 20 μm/secat 20° C. This rapid mobility allows motile sperm, but not thenon-motile sperm, to distribute themselves randomly within the width ofa 500 μm channel within seconds. The sorting device was designedspecifically to give sperm a residence time of 20 seconds in the mainseparation channel. A bifurcation placed at the end of this separationchannel allows efficient collection of only the motile sperm thatdeviated from its initial inlet stream.

The sorting device described integrates all functions necessary forsperm sorting, for example, inlet/outlet ports, fluid reservoirs, pumps,power source, sort channel, etc., onto a simple chip design that ispractical to manufacture and use. A key design feature of thisembodiment is the set of four horizontally-oriented fluid reservoirsthat also function as sample inlet/outlet ports and a fluid pumpingsystem. The orientation, geometry, and size of these reservoirs aredesigned to balance gravitational forces and surface tension forces, andprovide a pumping system that generates a steady flow rate over extendedperiods of time regardless of the volume of fluid in the reservoirs.This contrasts with conventional gravity-driven pumping systems whoseflow rates decrease over time as the volume of fluid in the inletreservoir decreases. The diameters of the reservoirs were selected to besmall enough that surface tension prevents liquid from spilling out ofthe horizontally-oriented reservoirs, but large enough to holdsufficient amounts of sample (tens to hundreds of microliters) and allowconvenient sample introduction and recovery. This balance of forcesallows the reservoirs to be arranged horizontally without the liquidinside spilling out. The horizontal reservoir arrangement, in turn,holds the height difference between the fluid in the inlet and outletreservoirs the same (1.0 mm height difference between inlet and outletreservoir ceilings) regardless of the volume of fluid present in thereservoirs and maintains a constant hydraulic pressure even as theamount of fluid in the reservoirs changes.

The passively-driven pumping system described here is unique in that ituses horizontally-oriented reservoirs to overcome the problem oftraditional gravity-driven pumping, where the pressure decreases as theamount of liquid in the reservoir decreases. Furthermore, the structureof the pump is greatly simplified compared to other mechanical ornon-mechanical pumping systems allowing easy manufacture and integrationof the pump into a small, integrated device. Finally, the use of gravityand surface tension as the driving-force contributes to the overallsmall size of the sorting device by eliminating the need for powersupplies such as batteries. Taking gravity, surface tension, and channelresistance into consideration, the sorting device was designed to give asteady flow rate of sperm with a residence time of approximately 20seconds inside the main sort channel. More specifically, the device isdesigned so that the flow resistance of the fluid reservoirs is morethan 10 times less than that of the microfluidic channels, and thereforenegligible. Thus, the resistance of the channels, calculated to be2.8×10¹² kg/(sec/m⁴), approximates the total resistance of the system.Since a bulk flow rate of 0.008 μL/sec is required to achieve thedesired residence time of 20 seconds and the total resistance is2.8×10¹² kg/(sec/m⁴), the net pressure drop required to drive the fluidis 23 N/m². To achieve this desired pressure drop, we designed thedimensions of the reservoirs such that capillary forces (3.0 mm diameterinlet reservoir vs. 2.0 mm diameter outlet reservoir) would be 13 N/m²and the pressure drop across the microfluidic channel of the sortingdevice due to hydrostatic forces (1.0 mm height difference) would be 9.8N/m². For calculation of the capillary force, the contact angle wasassumed to be 0° (the contact angle of water on BSA coated PDMS is verysmall), the surface tension of the washed semen sample assumed to beapproximately 0.040 N/m (less than that of water due to “impurities”such as proteins), and the viscosity of the washed semen sample to besimilar to that of water. The observed bulk flow rate of 0.008 μL/secfor a dilute particle suspension in 1% BSA solution was approximatelyequal to that of the calculated flow rate. Actual sperm samplessometimes had lower flow rates due to larger apparent viscosity. Smallerflow rates for the sperm sample stream would result in slightly loweryields but does not affect the purity of the sperm recovered at thesorted sperm outlet.

Sperm sorting efficiencies of the sorting device were evaluated by threemethods: (i) tracking the movement of motile sperm in the channel byphase contrast microscopy, (ii) tracking movement of propidium iodide(PI) stained cells in the channel by fluorescence microscopy, (iii)using a Makler Counting Chamber, a grid-based sperm counting device, todetermine numbers of motile sperm and non-motile sperm in the inlet andoutlet reservoirs (FIG. 8). The sperm tracking experiments shows theprocess of how motile sperm can swim out of its initial streamline. PIstains membrane compromised cells such as dead cells, and thus allowsthe non-motile sperm to be highlighted and visualized with redfluorescence while the motile sperm remain unstained. The bar graphs inFIG. 8 compare percentage of sperm that are motile before and aftersorting. The unshaded bars represent the initial sperm sample, while thesolid bars represent the motile particle-enriched media stream. Thepurity of motile sperm after sorting was nearly 100% regardless ofmotile sperm purity before sorting. The yields (39%, 42%, 43%), definedas the ratio of the number of motile sperm in the motile sperm outletreservoir to the total number of motile sperm in the sperm sample inletreservoir, were comparable to or greater than the recovery rates (0.8%to 50%) of sperm processed using conventional sorting methods such asdirect swim-up, swim-up from a pellet of centrifuged sperm, or densitygradient separation. It was also observed that sperm morphology, anotherimportant trait that correlates with successful pregnancies, alsoimproved after sorting with the device (Strict Sperm Morphology:9.5±1.1% normal before sorting to 22.4±3.3% normal after sorting).Kruger Strict sperm morphology is a set of criteria or standards wherebysperm must fit within specific measurements (head width and length, taillength, acrosome making up a certain percentage of the sperm head) andlack abnormalities (e.g. pin head, round head, crimped tail).

As can be seen from the above, the motile particle sorting devices aresmall, easily manufactured, simple in operation, and highly efficient.In the claims which follow, the terms “a” and “an” mean “one or morethan one” unless indicated otherwise.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A process for sorting motile sperm from lessermotile and/or non-motile sperm, comprising: a) providing a sort fluidcomprising motile sperm and at least one of lesser motile sperm ornon-motile sperm; b) providing a media fluid to be enriched with motilesperm; c) contacting a stream of said sort fluid and a stream of saidmedia fluid within a sort channel of a sorting device, wherein saidsorting device comprises i) a sort channel having first and second ends,wherein said sort channel has a length of less than 5 cm; ii) a sortfluid reservoir in fluid communication with said first end of said sortchannel; iii) a media fluid reservoir in fluid communication with saidfirst end of said sort channel; iv) a motile particle-depleted sortfluid outlet in fluid communication with said second end of said sortchannel; and v) a motile particle enriched media fluid outlet in fluidcommunication with said second end of said sort channel; said sort fluidstream and said media fluid stream having there between a non-turbulentinterface within said sort channel, wherein said media fluid reservoiracts as a gravity flow pump to transport said media fluid through saidsort channel, whereby motile sperm leave the sort fluid stream and enterthe media fluid stream by active movement, forming a motileparticle-depleted sort fluid stream and a motile particle-enriched mediafluid stream, wherein said sperm move perpendicular to gravity andperpendicular to the direction of flow of said media fluid in said sortchannel during said step of whereby said motile sperm leave the sortfluid stream and enter the media fluid stream, and d) separating amotile sperm-depleted sort fluid outlet stream from a motilesperm-enriched media fluid outlet stream to isolate motile sperm,wherein said motile sperm are alive.
 2. The process of claim 1, whereinsaid sort fluid is provided in a sort fluid reservoir and said mediafluid is provided in a media fluid reservoir.
 3. The process of claim 1,wherein said media fluid and said sort fluid flow in the same directionand at substantially the same flow rate.
 4. The process of claim 1,wherein said sort fluid stream and said media fluid stream exhibitlaminar flow within said sort channel.
 5. The process of claim 1,wherein the transverse volume of said media fluid stream is greater thanthe transverse volume of said sort fluid stream in said sort channel. 6.The process of claim 1, wherein said motile sperm depleted sort fluidoutlet stream comprises a sort fluid inlet stream to a second devicehaving a sort channel, and steps a) through d) are repeated, wherebyfurther motile sperm enter a media stream of said second device tofurther deplete said sort fluid of motile sperm.
 7. The process of claim1, wherein the purity of motile sperm after sorting is between 90% and100%, wherein said percent is based on the number of motile sperm insaid motile particle-enriched media outlet stream compared to totalsperm in said motile particle-enriched media outlet stream and saidmotile particle-depleted sort fluid stream.